B32B-01 10:25h
Geological Setting of Diamond Drilling for the Archean Biosphere Drilling Project, Pilbara Craton, Western Australia
The Archean Biosphere Drilling Project (ABDP) is a collaborative international research project conducting systematic (bio)geochemical investigations to improve our understanding of the biosphere of the early Earth. The Pilbara Craton of Western Australia, which includes exceptionally well preserved 3.52 to 2.70 Ga sedimentary sequences, was selected for an innovative sampling program commencing in 2003. To avoid near-surface alteration and contamination effects, sampling was by diamond drilling to depths of between 150 and 300 m, and was located at sites where the target lithologies were least deformed and had lowest metamorphic grade (below 300°C). The first of five successful drilling sites (Jasper Deposit) targeted red, white and black chert in the 3.46 Ga Marble Bar Chert Member. This chert marks the top of a thick mafic-felsic volcanic cycle, the third of four such cycles formed by mantle plumes between 3.52 and 3.43 Ga. The geological setting was a volcanic plateau founded on 3.72 to 3.60 Ga sialic crust (isotopic evidence). The second hole (Salgash) was sited on the basal section of the fourth cycle, and sampled sulfidic (Cu-Zn-Fe), carbon-rich shale and sandstone units separated by flows of peridotite. The third hole (Eastern Creek) was sited on the margin of a moderately deep-water rift basin, the 2.95 to 2.91 Ga Mosquito Creek Basin. This is dominated by turbidites, but the sandstones and carbon-rich shales intersected at the drilling site were deposited in shallower water. The fourth and fifth holes, located 300 km apart, sampled 2.77 to 2.76 Ga continental formations of the Fortescue Group; both holes included black shales.
B32B-02 10:40h
Origins of Carbonaceous Matter, Hematite, and Pyrite in the 3.46Ga Marble Bar Chert/Jasper/Basalt Formation, Pilbara, Western Australia
The 3.46 Ga Marble Bar Chert/Jasper unit in the Pilbara district, W.A. was probably deposited in a deep ($<$500 m) ocean during the accumulation of a thick ($<$5 km) submarine basalt. Debate focuses on whether organic matter in pre-2.0 Ga cherts is a remnant of marine organisms or the product of abiotic synthesis in hydrothermal systems, whether the hematite crystals in jaspers were primary or products of modern oxidation of siderite and pyrite, and whether the pyrite crystals formed by sulfate-reducing bacteria, hydrothermal fluids, or atmospheric sulfur. At the drilling site, the Marble bar Chert/Jasper is over turned diping about 80 degrees. A continuous 264 m- long core, drilled at 50 degrees, was recovered. The major chert/jasper unit, comprising alternating beds (0.1 - 5 cm thick) of white/green/gray/black/red chert, is 105 m thick. The abundance of red jasper beds increases down hole, indicating that the hematite crystals were not produced by subaerial oxidation of ferrous minerals. Petrological, mineralogical, and geochemical investigations of the core samples, especially using an X-ray chemical microscope, have revealed that the dominant Fe-bearing minerals are siderite, magnetite, and hematite, in the green, gray-black, and red cherts, respectively. These Fe-bearing minerals and disseminated pyrite crystals (ubiquitous in all cherts) are typically very fine grained (less than 10 microns). The basalts (pillows and tuffs), which occur below, above, and interbedded with the chert/jasper unit, are in places heavily hematitized with various characteristics of submarine hydrothermal alteration, such as chloritization, silicification, pyritization, large variations in the contents of Fe, Mg, Ba, and depletions in Ca, Sr, and Na. Veinles containing quartz and pyrite are abundant in the chert/jaster beds and also in the heavily hematitized basalts. These data suggest the hematite, magnetie, siderite, pyrite and silica in the chert/jasper unit, basalt tuffs, and pillow margins formed by rapid mixing of Fe-, H2S- and silica rich hydrothermal fluids with O2- and carbonate-rich deep ocean water. The presence of distinct negative Ce anomalies in the altered basalts also indicates reaction with oxygenated seawater. Evaluation of the relationship between the atmospheric and ocean water chemistry suggests an oxygen-rich ($<$0.5 PAL) and CO2-rich ($<$100 PAL) contemporaneous atmosphere. Carbonaceous matter in the chert/jasper unit typically occurs in thin layers ($<$1 to 5 mm in thickness) parallel to the bedding plane, commonly in clay-rich layers. Significant enrichment of uranium is observed in some organic C-rich and clay-rich layers, suggesting the reduction of U6+ ions in seawater by the carbonaceous matter during quiescent periods of submarine hydrothermal activity. These features suggest the carbonaceous matter was not generated by abiotic hydrothermal reactions, but is a remnant of microbial mats and the modern-style uranium geochemical cycle was already established 3.46 Ga ago.
B32B-03 10:54h
New Discoveries From The Archean Biosphere Drilling Project (ABDP)
The Archean Biosphere Drilling Project (ABDP), an international scientific drilling project involving scientists from the USA, Australia and Japan, was initiated in Pilbara Craton, Western Australia. The scientific objectives of the ABDP are the identification of microfossils and biomarkers, the clarification of geochemical environment of the early Earth, and the understanding of geophysical contribution to the co-evolution of life and environment. Through 2003 and 2004 activities, we have drilled 150 _| 300 m deep holes to recover _gfresh_h (modern weathering-free) geologic formations that range from 3.5 to 2.7 Ga in age. The drilling targets were: (1) 3.46 Ga Towers Formation, (2) mid-Archean Mosquito Formation, (3) 2.77 Ga Mt Roe Basalt, (4) 2.76 Ga Tumbiana Formation, (5) 2.74 Ga Hardey Formation. The initial investigations on the ABDP drill cores by Japanese members have already produced many exciting and interesting data and observations. 3.46 Ga Marble Bar Jasper could provide clues to the argument about the early photosynthetic cyanobacteria that have produced free oxygen and have evolved the oxygen level on the earth. There have been many ideas how the hematite in jasper was formed. Our most important discoveries are the confirmations that hematite, magnetite and siderite precipitated separately as primary minerals, and that there is a remaining texture which resembles microfossil using FE-SEM, ESCA, Laser-Raman and cathodoluminescence. Taking into account the carbon isotopic ratios of remains from _|25 to _|40 permil, these iron oxides might be biogenic. We need to identify the iron bacteria in detail to deduce the early earth_fs surface environment. In addition, the black shale of Apex Basalt overlying Marble Bar Jasper contains organic carbon from 0.7 to 5.2 percent, and the carbon isotopic ratio of which is from -26 to -30 per mil, suggesting that various microbes inhabited in the early Archean ocean. 2.77 Ga Mt Roe Basalt, which is composed of basaltic lavas interbedded with tuffs, clastic sediment and minor evaporites, well preserves the primary biogeochemical, geochemical and geophysical phenomena. The discovery of black shale with sulfide nodules is worthy of special attention. Our study suggests that the following succession of events occurred more than once, (1) eruption of amygdaloidal basaltic lava followed by eruption of tuff into shallower water, (2) deposition of sandstone and black shale, and (3) concurrent hydrothermal activity with reduced fluids altered the tuff and the lowermost clastic sediments. The extremely light carbon isotopic ratios suggest the activities of methanogene in hydrothermal veinlets and methanotroph in black shale. In addition, the wide range of sulfur isotopic ratio in black shale suggests activity of co-existing sulfate-reducing bacteria in the black shale. Occasional presence of sandstone, especially in late stage of clastic sedimentation, suggests the sedimentation near coastal environment. Stromatolite-like microtexture in the sandstone suggests the existence of photosynthetic microbes, which is supported by heavy carbon isotopic ratios (up to _|25 permil) and by the signals of hopanoids biomarker. The three dimensional geochemical data suggest the existence of marine environment from oxic at shallow site to euxinic at the deeper site. Paleomagnetic analyses suggest the episodic initiation of the earth's dynamo at about 3.5 Ga and the increase of it's momentum since at least 2.77 Ga. Taking into account the biogeochemical evidences confirmed from other ABDP cores, the increase of geomagnetic intensity might have accelerated the diversification of early life.
B32B-04 INVITED 11:08h
2004 NAI-ADP Deep Diamond Drill Cores: Transects Through Archean Time in the Pilbara Craton, Australia
In July-August 2004, the NASA Astrobiology Drilling Program sponsored the coring of 3 deep diamond-drill holes in the Pilbara Craton of northwest Australia. The holes targeted the lowest grade and least deformed sedimentary sections of 4 stratigraphic units: the 2.4-2.6 Ga Hamersley Group, the 2.7 Ga Tumbiana Formation of the Fortescue Group, the 3.4 Ga Warrawoona Group, and the 3.5 Ga Coonterunah Group. ABDP 8 cored the unconformity between the Warrawoona and Coonterunah Groups to a depth of 330 metres, intersecting it at 155 metres. Because of syn-depositional erosion, the Strelley Pool Chert was attenuated and the alteration zone beneath the unconformity was scoured and filled to a depth of 10 metres by quartz arenite. As a result, no definitive lithological determination on its status as a potential paleosol could be made. Secondary oxidative alteration was present in Coonterunah cherts to depths of at least 220 metres down-hole. ABDP 9 cored 984 metres of the lower Hamersley Group, from the Dales Gorge Member of the Brockman Iron Formation into the Paraburdoo Member of the Wittenoom Formation. Though the hole was intended to penetrate the uppermost Fortescue Group, drilling was terminated early because of equipment damage by fractured rock, loss of water circulation clogging the hole with cuttings and unanticipated thickening of the Paraburdoo Member by dilational fracturing, expansive brecciation and cavity formation. 79 samples for organic geochemical analysis of biomarker syngenesis were collected under clean conditions immediately the core surfaced. A horizon of impact spherules was intersected in the Bee Gorge Member of the Wittenoom Formation; unlike surface exposures, it was markedly silicified and chloritized in drill-core. ABDP 10 cored 210 metres of the Tumbiana Formation, intersecting the entire Meentheena Carbonate Member, the upper Mingah Tuff Member and terminating just below 4 scoriaceous basalt flows. Large and complex calcareous stromatolites, which were partly silificified in surface exposures, contained no chert in drill-core. Ooid grainstone was a prominent lithology, attesting to a shallow subaqueous depositional environment. Pseudomorphs possibly after diagenetic gypsum were encountered in a fine tuffaceous wacke unit, but no other evaporite minerals were evident. 8 samples for biomarker analysis were collected. The cores will be archived at the Geological Survey of Western Australia core library in Perth. They will be cut using non-magnetic saw blades to allow paleomagnetic studies. Quarter-core splits will be sent to the USA and Japan for detailed examination.
B32B-05 INVITED 11:22h
Hydrocarbon Analysis of Hamersley Basin Deep Drill Cores: Preliminary Results
Characterization of the organic constituents of Precambrian rocks is complicated by serious questions as to the provenance of organic material recovered from such ancient and generally altered deposits. The problem is particularly acute in Archean terranes where the rocks are invariably metamorphosed to some degree. Organic matter indigenous to the original depositional environment of the host sediments can be difficult to distinguish from material that subsequently migrated into the rock from other sources. Further, contamination by ubiquitous petroleum products and derivatives can easily be introduced at any point during sample recovery and preparation. Nevertheless, analysis of the molecular and isotopic signatures of Archean biomarker compounds has the potential to provide key insight into ancient biology and ecology. Several continental drilling efforts in recent years have concentrated on obtaining organic-bearing sedimentary samples of low metamorphic grade from Archean and Paleoproterozoic terranes while minimizing sources of contamination. Experimental techniques have been developed to avoid laboratory contamination and to assess the origins of various types of organic matter. Using ultraclean protocols, and a combination of molecular, isotopic and geologic evidence, confident assessments can be made as to the syngeneity of biomarkers in Archean rocks. We present results of analyses of drill core collected from the Hamersley Basin, Western Australia in the summer of 2004.
B32B-06 INVITED 11:37h
The Pilbara Drilling Project (PDP): A French-Australian Connection
The Pilbara Drilling Project (PDP) is a French-Australian research project organized by Pascal Philippot and his colleagues at the Institut de Physique du Globe de Paris (IPGP) and the Universite Paris-Sud and Martin Van Kranendonk and his associates from the Geological Survey of Western Australia (GSWA). The PDP has been carried out during August 2004 through funding from the IPGP and the French National Center for Scientific Research (CNRS). The objectives of the PDP are to investigate "fresh" Archaean rocks recovered from relatively shallow drilling performed below the water table in order to gain information on the Early Earth environments, with special emphasis on the chemistry and possibly temperature of the oceans and atmosphere, the nature and diversity of the biosphere and the dynamics of the mantle-crust-ocean-atmosphere system. Two cores of about 200 m each have been recovered in two different sites. The sedimentary and hydrothermal chert-barite horizon of the Dresser Formation (North Pole Dome; 3.49 Ga) and the shallow-marine or -lake carbonate sediments and tuffaceous material of the Tumbiana Formation (2.7 Ga).Special care has been taken on minimizing contamination by organic compounds during and after drilling and on core orientation.
B32B-07 11:52h
Multidisciplinary Study of the Precambrian Biosphere and Surficial Oxygenation, Kaapvaal Craton, South Africa: The Agouron Cores
The Campbellrand-Kuruman carbonate-iron formation stratigraphic succession, which drapes the Kaapvaal craton of South Africa, provides a unique opportunity to study the latest Archean/Earliest Proterozoic time interval in a multidisciplinary fashion, for four principal reasons: 1) The <1 km-thick succession of carbonates, cherts, shales, and associated iron formations is a storehouse of various geochemical and paleoclimatic proxy records, 2) the carbonate platform has never been significantly buried and contains abundant limestone, thus offering strong potential for preservation of organic biomarkers, 3) the occurrence of early chert and abundant early sea-floor carbonate crusts provide good potential for the preservation of microfossils and magnetofossils, and 4) much of the stratigraphic succession has not been significantly deformed and we have estabilshed a chronostratigraphic framework in which shallow water facies can be traced down the ancient paleoslope into facies deposited at water depths < 250 meters within a sequence stratigraphic context, supplemented with correlation of three impact spherule layers. The geologic framework provided by this sequence of rock offers an unparalleled opportunity to study the structure and composition of the Archean ocean and to merge this information with co-existing paleontological and geochemical records. With support from the Agouron Institute, two separate cores, each ~ 1.5 km in length, were drilled through the margin of the carbonate platform, spaced so as to intercept the transitional facies at two paleodepths. The holes were deviated slightly from vertical so that a ball-mark system could be used to obtain absolute orientation. To enhance the utility for paleomagnetic investigations, core barrels and bits were demagnetized routinely with a portable mu-metal shielded coil assembly to reduce remagnetization problems, and all core slicing was done with non-magnetic blades. To minimize contamination problems for geochemical and isotopic analyses, we avoided the use of drilling lubricants containing organic materials. Almost complete core recovery was obtained, and a variety of studies are now in progress. In late 2004/early 2005, two additional short holes will be drilled in the Koegas Subgroup of the Transvaal Supergroup, designed to cover gaps in the time period critical to the rise of atmosphereic oxygen (~2.45-2.22 Ga). This major gap in the succession in South Africa lies between the Kuruman-Griquatown iron-formation and Duitschland-Timeball Hill successions, and should be exposed in the subsurface in the area immediately west of Griquatown where the first two Agouron holes were drilled.
http://general.rau.ac.za/geology/geobiology/default.asp
B32B-08 12:06h
High-resolution C-N-S-Fe isotope chemostratigraphy of the terminal Proterozoic Huqf Supergroup, Sultanate of Oman: reorganization of global biogeochemical cycles and the progressive oxygenation of the ocean
We present a high resolution study of the terminal Neoproterozoic to Cambrian Huqf Supergroup of Oman using drill core and cuttings. The Huqf Supergroup overlies approximately 800Ma crystalline basement and is composed in ascending order of siliciclastics and glacio-marine diamictites (Ghadir Manquil Formation), carbonate-siliciclastic packages (Masirah Bay, Khufai, Shuram, and Buah Formations), and a thick carbonate-evaporite sequence (Ara Group). The ratio of highly reactive iron to total iron and the presence of significant Fe-carbonate, Fe-magnetite, and Fe-(oxyhydr)oxide species in the Neoproterozoic Huqf sediments indicates deposition under an anoxic ferrous water column, rather than under oxic or euxinic conditions. The speciation of highly reactive iron changes approaching the Precambrian/Cambrian boundary, suggesting a change in water column redox. Carbonate-associated sulfate (CAS) from these units is found to increase in abundance from less than 100pm in the lower units to several thousand ppm in the Ara Group. CAS d34S indicate a general increase up to 39 permil at the Precambrian/Cambrian boundary, followed by a decrease at the top of the Ara Group. The coincidence of CAS d34S and anhydrite d34S from the Ara Group provide confidence that the observed CAS d34S isotope signal is primary. The rise in sulfate d34S observed during terminal Proterozoic time and subsequent steady decline across the Precambrian/Cambrian boundary suggests a progressive shift in the balance of sulfur burial from sulfides to sulfates. There are two significant negative excursions in carbonate d13C from this section: an 8 permil excursion at the Precambrian/Cambrian boundary; and a 13 permil excursion in the Shuram Formation. The latter excursion extends through $<$500m of section, suggesting a long term perturbation to the C-cycle. Both excursions have correlative representations in globally distributed sections, improving confidence that our observations reflect significant, global changes to biogeochemical cycling of carbon during the terminal Neoproterozoic. Here we present organic d13C and d15N with a focus on outlining their behavior during these excursions. Taken together, these data indicate a massive reorganization of the global C-N-S-Fe biogeochemical cycles likely due to changing redox conditions (e.g., oxygenation) of the deep ocean.